Fluorescence and two-photon absorption of push-pull aryl(bi)thiophenes: Structure-property relationships

Article abstract of DOI:10.1039/c2pp25258a

Photophysical and TPA properties of series of push-pull aryl(bi)thiophene chromophores bearing electron-donating (D) and electron-withdrawing (A) end-groups of increasing strength are presented. All compounds show an intense intramolecular charge transfer

Full text of DOI:10.1039/c2pp25258a

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PAPER
Fluorescence and two-photon absorption of push–pull aryl(bi)thiophenes:
structure–property relationships†‡
Emilie Genin,a Vincent Hugues,a Guillaume Clermont,a Cyril Herbivo,b,c M. Cidália R. Castro,b Alain Comel,c
M. Manuela M. Raposob and Mireille Blanchard-Desce*a
Received 19th July 2012, Accepted 31st August 2012
DOI: 10.1039/c2pp25258a
Photophysical and TPA properties of series of push–pull aryl(bi)thiophene chromophores bearing
electron-donating (D) and electron-withdrawing (A) end-groups of increasing strength are presented. All
compounds show an intense intramolecular charge transfer (ICT) absorption band in the visible region.
Increasing the D and/or A strength as well as the length of the conjugated path induces bathochromic and
hyperchromic shifts of the absorption band as reported for analogous push–pull polyenes. Yet, in contrast
with corresponding push–pull polyenes, a significant increase in fluorescence is observed. In particular,
chromophores built from a phenyl–bithienyl conjugated path and bearing strong D and A end-groups
were found to combine very large one and two-photon brightness as well as strong emission in the
red/NIR region. These molecules hold promise as biphotonic fluorescent probes for bioimaging.
localised two-photon photodynamic therapy (TP-PDT),12 loca-
1. Introduction
lised photorelease of bioactive species,13 3D data storage,14
optical power limiting,15 and 3D microfabrication.9a,15f,16 In par-
Among the numerous classes of functional π-conjugated organic
materials used as organic semiconductors, oligothiophenes have
attracted widespread interest for many years1 due to their elec-
tronic/optical properties, high thermal and chemical stability and
their relative ease of synthesis. They have been used for the fab-
rication of (opto)electronic devices such as organic field-effect
transistors (OFETs),2 organic light emitting diodes (OLEDs),3
photovoltaic solar cells (OPVs and OSCs),2a,4 optically pumped
lasers5 and molecular nanowires.6 They have also been applied
in applications such as non-linear optics (NLO)7 and biology.8 In
particular, the past decade has been marked by a growing interest
in the design of NLO chromophores for two photon absorption
(TPA) applications. These efforts have been driven by the many
applications of the TPA phenomenon in various fields including
material science, biology and medicine.9 Owing to the possi-
bility it provides to use red–NIR excitation (allowing deeper pen-
etration in scattering materials and tissues), and to the quadratic
dependency of the TPA process on excitation intensity respon-
sible for an intrinsic three-dimensional spatial resolution, techno-
logical applications have been developed such as two-photon
excited fluorescence (TPEF) laser-scanning microscopy,10,11
ticular, the increased popularity of TPEF microscopy for biology
applications has motivated numerous efforts in the design
of optimised fluorophores having large two-photon brightness
(i.e. σ2Φ where σ2 is the TPA cross-section and Φ the fluo-
rescence quantum yield).9f,11e–h,15e,17 In addition, for easier
detection in biological media and tissues, red-emitting fluoro-
phores are particularly helpful. In this context, we have been
interested in investigating the potentialities of recently published
push–pull oligothiophene derivatives whose second-order optical
responses have been shown to be of interest.7c,f,18 Interestingly,
(push–pull) oligothiophene derivatives have been largely
described in the literature for organic solar cells purpose4a,d,19
but, to the best of our knowledge, only scarce examples of oligo-
thiophene derivatives have been investigated for two-photon20 or
three-photon21 excited photoluminescence up to now.
In this context, we herein report the detailed study of the fluor-
escence and TPA properties of series of push–pull aryl(bi)thio-
phene chromophores bearing various electron-donating (OR,
NR2) groups and electron-withdrawing (formyl, dicyanovinyl
and barbiturate) end-groups and either one or two thienyl units
in the conjugated path. This allowed us to derive structure–
property relationships aiming at optimisation of push–pull chromo-
phores for TPEF microscopy.
aUniv. Bordeaux, ISM, UMR 5255 CNRS, F-33400 Talence, France.
E-mail: mireille.blanchard-desce@u-bordeaux1.fr; Tel: +33 (5) 4000 6732
bCenter of Chemistry, University of Minho, Campus of Gualtar,
4710-057 Braga, Portugal. Tel: +351 253 604381
cUniversité de Lorraine, Institut Jean Barriol, Laboratoire de Chimie
et Physique - Analyse Multi-échelles des Milieux Complexes, F-57048
Metz Cedex, France
2. Results and discussion
2.1 Compounds
†This article is published as part of a themed issue in honour of Jean-
Pierre Desvergne on the occasion of his 65th birthday.
‡Electronic supplementary information (ESI) available. See DOI:
10.1039/c2pp25258a
All derivatives were easily obtained by Knoevenagel conden-
sations starting from 2-formyl-5-arylthiophenes or 5-aryl-5′-
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Scheme 1 Structure of compounds 1–6 and the neutral (N) and zwitterionic (Z) limiting-resonance forms.
formyl-2,2′-bithiophenes.7f,18a As reported earlier, 2-formyl-5-
2.2 Photophysical properties
arylthiophenes are easily obtained via the Vilsmeier–Haack–
Arnold reaction whereas formyl-substituted 5-aryl-2,2′-bithio-
phenes are easily obtained via the Suzuki coupling of functiona-
The photophysical properties of derivatives 1–6 are collected in
Table 1.
lised aryl boronic acids with 5
́-bromo-5-formyl-2,2́-bithio-
phene.18a
Absorption. The absorption spectra in solution of all deriva-
tives display an intense low-energy absorption band located in
the near UV or visible region depending on the strength of the
D and A end-groups as well as on the length of the conjugated
path. This strong band, associated with high molar extinction
coefficients (ranging from 2.2 to 6 × 104 mol−1 L cm−1), can be
ascribed to an intramolecular charge transfer (ICT) transition, as
is the case for analogous related push–pull polyenes.7g,22,23 In
the case of the red-shifted chromophores 4e, 5a–d and 6a–b, d,
e having the longer conjugated path and bearing the stronger
D/A pairs, a second lower-intensity higher-energy absorption
band located in the 350–450 nm range is also observed. This
additional band can be ascribed to a higher energy π–π*
transition.
In the present investigation, different formyl precursors 1a–d
and 3a, c–e bearing different electron-donating end-groups (D)
and having arylthiophene (1) or arylbithiophene (3) π conjugated
linkers7f,18a were selected in order to evaluate the influence of
the electronic strength of the D substituents and of the length of
the π-conjugated bridge on the fluorescence and TPA properties
of these heteroaromatic aldehydes (Scheme 1). In order to
increase the push–pull character of the derivatives, we also
shifted to popular strong electron-withdrawing (A) end-groups
such as dicyanovinyl (DCV) and 1,3-diethyl-2-thioxodihydro-
pyrimidine-4,6-dione also known as 1,3-diethyl-2-thiobarbiturate
(DETB). Such A end-groups have been shown to lead to very
large optical non-linearities (both second- and third-order)7g,22
as well as long distance photo-induced charge transfer23 in
push–pull polyenes having dialkylamino donating end-groups.
The dicyanovinyl derivatives 2 and 4 (already reported7f) and
DETB derivatives (series 5 and 6) were prepared in a similar
way as related push–pull polyenes7g,22 by reacting aldehyde
precursors 1 and 3 with malononitrile or 1,3-diethyl-2-thiobarbi-
turic acid in refluxing ethanol, in the presence of a catalytic
amount of piperidine (Scheme 2). New derivatives 5 and 6 were
End-groups effect on the ICT absorption band. As previously
observed for series 1–4, comparison of the series of thiobarbitu-
ric acid derivatives 5 and 6 shows that increasing the electron-
releasing strength of the D end-group (H → OMe → NEt2)
induces a marked red-shift of the absorption band (Fig. 1). Simi-
larly increasing the electron-withdrawing strength of the A end-
group (CHO → DCV → DETB) in series 1, 2, 5 leads to a sig-
nificant bathochromic and hyperchromic shift as well as marked
narrowing of the main absorption band (Fig. 1). The same trend
is also observed for the bithienyl series 3, 4 and 6. The batho-
chromic and hyperchromic effects as well as the narrowing of
the ICT band induced by increasing either D or A strength indi-
cate that the polarisation increases in the ground state (corre-
sponding to an increase of the contribution of the zwitterionic
mesomeric form (Z) in the description of the electronic distri-
bution) shifting the electronic structure towards a more “cyanine-
like” distribution (Scheme 1).24 This behaviour is similar to that
observed for related push–pull polyenes (i.e. having NR2 donat-
ing end-groups and DCVor TETB acceptor end-groups).7g,22
1
characterised by H and 13C NMR, IR, elemental analyses and
HRMS.
Scheme 2 Synthesis of thiobarbituric acid derivatives 5 and 6.
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Table 1 Photophysical properties of compounds 1–6 in chloroform solution
max
max
λabs
Log εmax
FWHM
λem
Stokes shift
kr
τ (ns) (108 s−1
knr
D
H
A
π
(nm)
(M−1 cm−1
)
(103 cm−1
)
(nm)
(103 cm−1
)
Φf
0.4
)
(109 s−1
)
(%)
1a
CHO
CHO
Ph–Thien
Ph–Thien
Ph–Thien
331
4.35
4.34
4.45
4.54
4.56
4.54
4.55
4.39
4.38
4.45
4.40
4.59
4.61
4.59
4.71
4.74
4.72
4.79
4.72
4.74
4.66
4.6
4.6
4.3
3.8
3.9
3.9
3.5
4.4
4.5
4.8
4.6
3.8
3.9
4.0
3.2
3.2
3.2
2.9
3.2
3.3
3.8
375
427
507
443
502
502
609
469
505
601
612
546
599
727
524
555
561
669
596
644
781
3.6
5.0
4.8
2.2
3.3
3.2
3.1
4.3
5.0
6.2
6.0
3.0
3.9
4.4
2.4
2.3
2.4
2.6
2.2
3.0
4.2
—
—
—
1b OMe
1d NMe2 CHO
352
408
404
431
433.5
511.5
13.7
84.5
0.04
0.2
0.2
13.5
35
79
77
74
0.5
5
0.6
2.9
—
—
—
0.6
1.2
2.5
3.4
3.4
—
0.4
2.7
—
—
—
0.4
—
0.2
1.1
2.3
2.9
—
—
—
2.1
3.0
3.2
2.3
2.2
—
1.4
1.2
—
—
—
0.22
—
0.44
0.05
1.41
0.06
—
—
—
1.37
0.55
0.09
0.07
0.08
—
2.64
0.24
—
—
—
2.28
—
4.56
0.84
2a
H
CHvC(CN)2 Ph–Thien
CHvC(CN)2 Ph–Thien
CHvC(CN)2 Ph–Thien
2b OMe
2c OEt
2d NMe2 CHvC(CN)2 Ph–Thien
3a
H
CHO
CHO
Ph–(Thien)2 390.5
Ph–(Thien)2 403
Ph–(Thien)2 437.5
Ph–(Thien)2 448.5
3c OEt
3d NMe2 CHO
3e NEt2 CHO
4a
H
CHvC(CN)2 Ph–(Thien)2 468
CHvC(CN)2 Ph–(Thien)2 486.5
CHvC(CN)2 Ph–(Thien)2 551.5
DETB
DETB
DETB
4c OEt
4e NEt2
34
0.04
5a
H
Ph–Thien
Ph–Thien
Ph–Thien
Ph–Thien
Ph–(Thien)2 526.5
Ph–(Thien)2 541
Ph–(Thien)2 587.5
464.5
491.5
494
5b OMe
5c OEt
5d NMe2 DETB
0.1
0.2
8.7
0.5
8.8
5.6
571.5
6a
H
DETB
DETB
6b OMe
6d NMe2 DETB
Fig. 1 UV-vis absorption spectra of compounds 5a–d and 1,2,5d: end-groups effect (donor strength: left and acceptor strength: right).
Length and connector effect. As expected, increasing the
length of the π-conjugated bridge also has a marked influence on
the photophysical properties. As an illustration, comparison of
related derivatives 5 and 6 bearing strong DETB emphasises that
increasing the number of thienyl moieties induces a marked
bathochromic shift of the absorption band (Fig. 2), as already
reported for push–pull derivatives bearing formyl or DCV accep-
tors (series 1, 3 and 2, 4 respectively). This bathochromic shift is
accompanied by a significant broadening in relation to a reduced
contribution of the zwitterionic mesomeric form due to the
higher cost of charge separation in bithienyl derivatives
(Scheme 1). Such a trend was also observed for related push–
pull polyenes.7g,22 Furthermore, comparison of the absorption
characteristics of push–pull (oligo)thienyl compounds 1d, 2d, 5d
bearing NMe2 electron-releasing substituents and those of their
polyenic analogues indicates that replacing two double bonds by
one thienyl unit in the conjugated path leads to a clear blue-shift
of the absorption maximum. Replacement of four double bonds
by two thienyl units has a similar effect as indicated by compari-
son of absorption data of compounds 3e, 4e, 6d and their polye-
nic analogues. This hypsochromic effect indicates a lower
Fig. 2 UV-vis absorption spectra of push–pull compounds 5–6,d:
effect of π-connector length.
delocalisation in oligothienyl push–pull derivatives (i.e. lower
contribution of the Z form as compared to analogous polyenes
having the same number of double bonds in the conjugated path)
in relation to the cost of charge separation associated with the
loss of aromaticity of the thienyl units (Scheme 1). We observe
that the largest effects are obtained for derivatives having
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aldehydes and DETB (i.e. 1415 cm−1 for compound 1d,
1290 cm−1 for compound 3e, 1900 cm−1 for compound 5d and
1850 cm−1 for compound 6d).
In contrast, considering both the thienyl 1, 2, 5 and bithienyl
3, 4, 6 series and comparing data for compounds having the
same D substituent but different A end-groups (1 → 2 → 5 or
3 → 4 → 6), we observe that increasing the A strength (CHO →
DCV → DETB) induces both a marked decrease of the Stokes
shift and a drastic decrease of the fluorescence quantum yields
(Table 1). The decrease of the Stokes shift values parallels the
narrowing of the ICT absorption band and emission and absorp-
tion red shifts and can be related to the shift of the electronic
structure towards the cyanine limit. The steady decrease of the
fluorescence quantum yield can be ascribed to the combined
effect of the reduction of the radiative decay rate (in connection
with the red-shifted emission)25 and the increase of the non-
radiative decay rates (see Table 1). In some cases (2d, 4c, 5d,
6b), the large non-radiative decay rates suggest the possible
onset of competing processes occurring in the excited state such
as photoinduced electron transfer.
Fluorescence. Interestingly, in contrast to analogous push–pull
polyenes,23b,d a number of derivatives from series 1–6,a–d show
significant fluorescence. This can be ascribed to the lower con-
formational stability of the (bi)thienyl π-linker as compared to
the polyenic chain, resulting in repression of efficient vibrational
deactivation processes.
As observed from Table 1, the fluorescence characteristics are
strongly dependent on the nature of the end-groups, as well as
on the length of the conjugated path.
End-groups effect. As was observed for absorption, increasing
either the electron-releasing or electron-withdrawing character of
the end-groups induces a marked bathochromic shift of the emis-
sion band (Fig. 3), in agreement with the ICT nature of the tran-
sition. Moreover the fluorescence quantum yield tends to be
higher for stronger donors. Only push–pull aldehydes 3 bent this
rule since they display high fluorescence quantum yield values
(up to 79%) regardless of the nature of the D end-group. As a
result, all derivatives having strong D end-groups (i.e. d,e deriva-
tives, D = NR2) show noticeable fluorescence, indicative of the
π–π* nature of the lowest energy excited state due to the ICT
character of the transition which shifts its energy below that of
the n–π* transition.
Length effect. Interestingly, the emission is significantly
red-shifted when the conjugated path is lengthened (phenyl–
thienyl → phenyl–bithienyl) as illustrated in Fig. 4 and indicated
by comparison of the corresponding chromophores of series 3,
4, 6 and 1, 2, 5. This red-shift is even more pronounced than that
for absorption resulting in very large Stokes shift values
(Table 1) and red or even NIR emission for bithienyl derivatives
combining good donor (OR, NR2) and acceptor (DCV, DETB)
end-groups. Even more strikingly, the increased length induces a
marked fluorescence enhancement: whatever the series, bithienyl
Fig. 3 Fluorescence emission spectra of push–pull compounds 5b–d and 1,2,5d: end-groups effect (donor strength: left and acceptor strength: right).
Fig. 4 Fluorescence emission spectra of push–pull compounds 1,3d; 2,4c and 5–6c: length effect.
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derivatives are found to be more fluorescent than their thienyl
analogues (except for the case of compound 6d whose low radia-
tive decay rate is responsible for a slight loss of fluorescence).
Hence, the bithienyl aldehyde 2a displays an intense fluor-
escence (fluorescence quantum yield value of 35%) even in the
absence of an electron-releasing para-substituent on the aromatic
ring, thanks to the combined donor and transmitter nature of the
thiophene moiety.
As a result of combined end-groups and length effects, push–
pull derivatives built from the bithienyl π-connector and bearing
good donor/acceptor end-groups (OMe, NR2/DCV, DETB: 4e,
6b,d) are found to be good red to NIR emitters.
whose maximum is located at about twice the wavelength of the
one-photon absorption (OPA) band, which indicates that the
lowest excited state (ICT transition) is both one and two-photon
allowed as expected for push–pull derivatives. In addition,
fluorescent derivatives 4e and 5–6d having strong D and A
groups show an additional TPA band at higher energy corre-
sponding to the less intense absorption band located in the near
UV (4e, 5d) or violet (6d) region. Interestingly, whereas the ICT
band is both one- and two-photon allowed, the second absorp-
tion band corresponding to a higher excited state is much more
strongly two-photon allowed, leading to high TPA cross-section
values in a wide spectral range of interest for biological imaging
(typically 1250 GM at 800 nm and 1570 GM at 1170 nm for the
red-emitting compound 5d). Such a phenomenon has already
been observed in octupolar merocyanines.28
2.3 Two-photon absorption
Based on their fluorescence, we were able to determine the TPA
characteristics of the derivatives 1b, 1d, 2b–d, 3, 4c, 4e, 5d and
6d–e by investigating their two-photon induced fluorescence in
solution. TPA spectra were obtained in the 700–1200 nm spectral
range through femtosecond two-photon excited fluorescence
experiments and by following the methodology described by
Webb and collaborators.26 The corresponding data are gathered
in Table 2. All compounds show a broad TPA band27 (Fig. 5)
End-groups effect. For the different series, comparisons of
chromophores bearing different electron-releasing groups
showed that increasing the donor strength leads both to a batho-
chromic shift of the TPA band (in connection with the ICT band
red-shift) and a marked enhancement of the maximum TPA
cross section (σ2) values (Table 2) as illustrated in Fig. 5. A
similar trend is obtained when increasing the acceptor strength
(Fig. 5). Interestingly, this hyperchromic effect is much more
Table 2 TPA properties of compounds 1–6 in chloroform solution
max1
max2
D
A
π
2λICT (nm)
λTPAmax1 (nm)
σ2 (GM) at λTPA
λTPAmax2 (nm)
σ2 (GM) at λTPA
1b
1d
2a
2b
2c
2d
3a
3c
3d
3e
4c
OMe
NMe2
H
OMe
OEt
NMe2
H
OEt
CHO
CHO
CHvC(CN)2
CHvC(CN)2
CHvC(CN)2
CHvC(CN)2
CHO
CHO
CHO
CHO
CHvC(CN)2
Ph–Thien
Ph–Thien
Ph–Thien
Ph–Thien
Ph–Thien
Ph–Thien
Ph–(Thien)2
Ph–(Thien)2
Ph–(Thien)2
Ph–(Thien)2
Ph–(Thien)2
704
816
808
862
867
1023
781
806
875
897
973
730
820
820
880
880
1050
770
820
890
56
124
28
—
—
—
—
—
—
—
—
—
—
—
—
790
—
800
—
—
—
—
—
—
—
—
—
—
—
—
—
—
780
—
1240
—
—
95
108
176
47
115
208
253
210
125
570
600
428
>1600
1180
NMe2
NEt2
OEt
890
930
1040
1100
1190
1120
>1170
1100
4e
NEt2
CHvC(CN)2
Ph–(Thien)2
1103
5d
NMe2
NMe2
DETB
DETB
Ph–Thien
Ph–(Thien)2
1143
1175
6da
a Not determined due to excitation and fluorescence wavelengths overlap.
Fig. 5 Two-photon absorption spectra of compounds 3a–e and 1,2,5d: end-groups effect (donor strength: left and acceptor strength: right).
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Fig. 6 Two-photon absorption spectra of compounds1,3d and 5–6d: length effect.
General procedure for the synthesis of arylthiophene and
arylbithiophenethiobarbituric acid derivatives 5–6 from the cor-
responding formyl precursors 1 and 3 by Knoevenagel conden-
sation. To a solution of 1,3-diethyl-2-thiobarbituric acid (80 mg,
0.38 mmol, 1.2 equiv.) and aldehydes 1 or 3 (0.32 mmol,
1.0 equiv.) in ethanol (20 ml) was added piperidine (1 drop).
The solution was stirred at reflux during different reaction times
(3–4 h) and cooled until room temperature. The mixture was
filtered and the crude solid was washed with diethyl ether.
Recrystallisation from dichloromethane–petroleum ether gave
the pure compounds.
pronounced for TPA than OPA in relation to the pronounced ICT
character of the transition.24b
Length effect. Finally the nature of the π-conjugated bridge
strongly influences the TPA properties of the compounds.
Increasing the number of thiophene moieties also induces a red-
shift of the TPA (paralleling the red-shift of the ICT absorption
band) and leads to a marked enhancement of the TPA cross-
section values (σ2) as illustrated in Fig. 6. As a result, push–pull
derivatives having two thienyl units and combining strong D and
A end-groups (typically D = NR2 and A = DCV or DETB) were
found to show very large maximum TPA cross-sections in the
spectral region of interest for bioimaging (1600 GM at 1200 nm
for NIR emitting compound 6d).
1,3-Diethyl-dihydro-5-((5′-phenylthiophen-2′-yl)methylene)-2-
thioxopyrimidine-4,6(1H,5H)-dione 5a. Orange solid (51%).
Mp 223–224 °C. IR(KBr) ν 3088, 1657, 1547, 1506, 1394,
1331 cm−1 1H NMR (DMSO-d6) δ 1.20 (m, 6 H, 2 ×
.
NCH2CH3), 4.44 (m, 4 H, 2 × NCH2CH3), 7.50 (m, 3 H, 3′′-H,
4′′-H and 5′′-H), 7.90 (m, 3 H, 2′′-H, 6′′-H and 4′-H), 8.31 (d,
1 H, J = 4.2 Hz, 3′-H), 8.66 (s, 1 H,vCH).13C NMR (CDCl3) δ:
12.4, 12.5, 43.2, 44.0, 110.4, 124.8, 126.8, 129.2, 130.0, 136.8,
147.2, 149.8, 152.1, 159.8, 161.0, 161.4, 178.7. Anal. Calcd for
C19H18N2O2S2: C, 61.60; H, 4.90; N, 7.56; S, 17.3. Found: C,
61.70; H, 5.04; N, 7.58; S, 16.9.
3. Experimental section
3.1 Compounds
The synthesis and characterisation of compounds 1–4,a–e have
been described earlier.7f,18a New compounds 5–6,a–e have been
synthesised following the same procedure.
Thin layer chromatography was carried out on 0.25 mm thick
precoated silica plates (Merck FertigplattenKieselgel 60F254).
Melting points were measured on a Gallenkamp melting point
apparatus. NMR spectra were obtained on a Varian Unity Plus
Spectrometer at an operating frequency of 300 MHz for 1H
NMR and 75.4 MHz for 13C NMR or a Bruker Avance III 400 at
an operating frequency of 400 MHz for 1H NMR and
100.6 MHz for 13C NMR using the solvent peak as an internal
reference at 25 °C. All chemical shifts are given in ppm using
δHMe4Si = 0 ppm as a reference and J values are given in Hz.
Assignments were made by comparison of chemical shifts, peak
multiplicities and J values and were supported by spin decou-
pling-double resonance and bidimensional heteronuclear HMBC
and HMQC correlation techniques. IR spectra were run on an
FTIR Perkin-Elmer 1600 spectrophotometer in nujol or in KBr.
Elemental analyses were carried out on a Leco CHNS 932
instrument. Low and high resolution mass spectrometry analyses
were performed at the “C.A.C.T.I. – Unidad de Espectrometria
de Masas”, at the University of Vigo, Spain. 1,3-Diethyl-2-thio-
barbituric acid was purchased from Sigma-Aldrich and used
without further purification.
1,3-Diethyl-5-((5′-(4′′-methoxyphenylthiophen-2′-yl)-methylene)-
2-thioxodihydropyrimidine-4,6(1H,5H)-dione 5b. Red solid (73%).
Mp 221–223 °C. IR(KBr) ν 3089, 2948, 1656, 1603, 1493,
1
1395, 1333, 1257, 1031 cm−1. H NMR (DMSO-d6) δ 1.20 (m,
6 H, 2 × NCH2CH3), 3.83 (s, 3 H, OCH3), 4.44 (m, 4 H, 2 ×
NCH2CH3), 7.06 (d, 2 H, J = 8.9 Hz, 3′′-H and 5′′-H), 7.79 (d,
1 H, J = 4.2 Hz, 4′-H), 7.86 (d, 2 H, J = 8.9 Hz, 2′′-H and
6′′-H), 8.28 (d, 1 H, J = 4.2 Hz, 3′-H), 8.64 (s, 1 H, CvCH).
13C NMR (DMSO-d6) δ 12.4, 12.5, 43.1, 43.9, 55.5, 109.5,
114.7, 123.9, 125.8, 128.4, 135.9, 147.7, 149.7, 159.9, 161.1,
161.4, 162.1, 178.7. Anal. Calcd for C20H20N2O3S3: C, 59.98;
H, 5.03; N, 6.99; S, 16.0. Found: C, 59.75; H, 5.08; N, 7.00;
S, 15.7.
1,3-Diethyl-5-((5′-(4′′-ethoxyphenylthiophen-2′-yl)-methylene)-
2-thioxodihydropyrimidine-4,6(1H,5H)-dione 5c. Violet solid
(58%). Mp 198–199 °C. IR(KBr) ν 3081, 2934, 1651, 1603,
1
1486, 1388, 1331, 1255, 1052 cm−1. H NMR (DMSO-d6) δ
1.20 (m, 6 H, 2 × NCH2CH3), 1.35 (t, 3 H, J = 6.9 Hz,
OCH2CH3), 4.10 (q, 2 H, J = 6.9 Hz, OCH2CH3), 4.44 (m, 4 H,
2 × NCH2CH3), 7.05 (d, 2 H, J = 8.6 Hz, 3′′-H and 5′′-H), 7.79
This journal is © The Royal Society of Chemistry and Owner Societies 2012
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(d, 1 H, J = 4.3 Hz, 4′-H), 7.86 (d, 2 H, J = 8.6 Hz, 2′′-H and
6′′-H), 8.29 (d, 1 H, J = 4.3 Hz, 3′-H), 8.64 (s, 1 H, CvCH).
13C NMR (DMSO-d6) δ 12.5, 14.7, 43.1, 43.9, 63.7, 109.4,
115.1, 123.9, 125.6, 128.4, 135.9, 147.8, 149.7, 159.9, 160.8,
161.1, 162.3, 178.7. MS (microTOF) m/z (%): 415 (M+, 100),
338 (36). HRMS: m/z (microTOF) for C21H23N2O3S2; calcd
415.1145; found: 415.1127. Anal. Calcd for C21H22N2O3S3: C,
60.84; H, 5.35; N, 6.76; S, 15.5. Found: C, 60.91; H, 5.30; N,
6.76; S, 15.3.
NMR (DMSO-d6) δ 1.22 (m, 6 H, 2 × NCH2CH3), 2.96 (s, 6 H,
N(CH3)2), 4.45 (m, 4 H, 2 × NCH2CH3), 6.75 (d, 2 H, J =
8.7 Hz, 3′′-H and 5′′-H), 7.44 (d, 1 H, J = 3.9 Hz, 4′-H), 7.58 (d,
2 H, J = 8.7 Hz, 2′′-H and 6′′-H), 7.68 (d, 1 H, J = 4.2 Hz, 3-H),
7.77 (d, 1 H, J = 3.9 Hz, 3′-H), 8.25 (d, 1 H, J = 4.2 Hz, 4-H),
8.62 (s, 1 H,vCH). MS (microTOF) m/z (%): 496 (M+ + 1, 45),
495 (M+, 20), 413 (17), 379 (13), 299 (91), 226 (5). HRMS: m/z
(microTOF) for C25H25N3O2S3; calcd 495.1109; found:
495.1103. Anal. Calcd for C25H25N3O2S3: C, 60.58; H, 5.08;
N, 8.48; S, 19.4. Found: C, 60.63; H, 5.08; N, 8.58; S, 18.9.
1,3-Diethyl-5-((5′-(4′′-N,N-dimethylaminophenylthiophen-2′-yl)-
methylene)-2-thioxodihydropyrimidine-4,6(1H,5H)-dione 5d.
Violet solid (81%). Mp 260–262 °C. IR(KBr) 3073, 2931, 1649,
5-((5-(5-(4-(Diethylamino)phenyl)thiophen-2-yl)thiophen-2-yl)-
methylene)-1,3-diethyl-dihydro-2-thioxopyrimidine-4,6(1H,5H)-
dione 6e. Green solid (96%). Mp 199–201 °C. IR(nujol)
ν 1684, 1655, 1605, 1551, 1539, 1516, 1481, 1411, 1395, 1356,
1307, 1269, 1250, 1231, 1217, 1159, 1108, 1096, 1077, 1054,
1019, 1001, 956, 925, 900, 881, 861, 813, 793, 779, 740, 735,
1
1604, 1488, 1387, 1362, 1329 cm−1. H NMR (CDCl3) δ 1.35
(m, 6 H, 2 × NCH2CH3), 3.11 (s, 6 H, N(CH3)2), 4.60 (m, 4 H,
2 × NCH2CH3), 6.70 (d, 2 H, J = 9.1 Hz, 3′′-H and 5′′-H), 7.40
(d, H, J = 4.3 Hz, 4′-H), 7.70 (d, 2 H, J = 9.1 Hz, 2′′-H and
6′′-H), 7.82 (d, 1 H, J = 4.3 Hz, 3′-H), 8.59 (s, 1 H, CvCH).
13C NMR (CDCl3) δ 12.4, 12.5, 40.1, 43.0, 43.8, 107.9, 112.0,
120.7, 122.8, 128.3, 134.8, 148.5, 149.2, 151.8, 160.0, 161.4,
164.6, 178.7. Anal. Calcd for C21H23N3O2S2: C, 60.99; H, 5.61;
N, 10.2; S, 15.5. Found: C, 60.97; H, 5.68; N, 10.1; S, 15.2.
1
720, 680, 651, 625, 610, 584, 547 cm−1. H NMR (DMSO-d6)
δ 1.10 (t, 6 H, J = 6.9 Hz, Ph-N(CH2CH3)2, 1.22 (m, 6 H, 2 ×
NCH2CH3), 3.38 (q, 4 H, J = 6.9 Hz, PhN(CH2CH3)2), 4.44 (m,
4 H, 2 × NCH2CH3), 6.71 (d, 2 H, J = 8.7 Hz, 3′′-H and 5′′-H),
7.40 (d, 2 H, J = 3.9 Hz, 4′-H), 7.54 (d, 2 H, J = 8.7 Hz, 2′′-H
and 6′′-H), 7.68 (d, 1 H, J = 3.9 Hz, 3′-H), 7.76 (d, 1 H, J =
3.9 Hz, 3-H), 8.25 (d, 1 H, J = 3.9 Hz, 4-H), 8.61 (s, 1
H,vCH). MS (microTOF) m/z (%): 524 (M+ + 1, 91), 523 (M+,
31), 497 (24), 453 (30), 393 (32), 349 (27), 318 (16). HRMS:
m/z (microTOF) for C27H30N3O2S3; calcd 524.1455; found:
524.1495. Anal. Calcd for C27H29N3O2S3: C, 61.92; H, 5.58;
N, 8.02; S, 18.4. Found: C, 61.62; H, 5.31; N, 7.99; S, 18.3.
1,3-Diethyl-dihydro-5-((5-(5-phenylthiophen-2-yl)thiophen-2-yl)-
methylene)-2-thioxopyrimidine-4,6(1H,5H)-dione 6a. Pink solid
(60%). Mp 240–241 °C. IR(nujol) ν 1685, 1655, 1548, 1528,
1495, 1414, 1328, 1301, 1266, 1171, 1152, 1105, 1073, 998,
972, 897, 806, 782, 759, 722, 759, 722, 691, 640, 608 cm−1
.
1H NMR (DMSO-d6) δ 1.22 (m, 6 H, 2 × NCH2CH3), 4.42 (m,
4 H, 2 × NCH2CH3), 7.43 (m, 3 H, 3′′-H, 4′′-H and 5′′-H), 7.66
(d, 1 H, J = 3.9 Hz, 4′-H), 7.77 (m, 3 H, 3′-H, 2′′-H and 6′′-H),
7.81 (d, 1 H, J = 3.9 Hz, 3-H), 8.28 (d, 1 H, J = 3.9 Hz, 4-H),
8.64 (s, 1 H,vCH). MS (microTOF) m/z (%): 453 (M+ + 1, 20),
452 (M+, 5), 338 (68), 316 (17), 288 (44), 239 (14), 197 (90),
175 (9). HRMS: m/z (microTOF) for C23H21N2O2S3; calcd
453.07204; found: 453.07597. Anal. Calcd for C23H20N2O2S3:
C, 61.03; H, 4.45; N, 6.19; S, 21.2. Found: C, 60.78; H, 4.23;
N, 6.15; S, 21.3.
3.2 Photophysical study
All photophysical studies have been performed with freshly-pre-
pared air-equilibrated solutions at room temperature (298 K).
UV/Vis absorption spectra of 10−5 M chloroform solutions were
recorded on a Jasco V-570 spectrophotometer. Steady-state and
time-resolved fluorescence measurements were performed on
dilute solutions (ca. 10−6 M, optical density < 0.1) contained in
standard 1 cm quartz cuvettes using an Edinburgh Instruments
(FLS920) spectrometer in photon-counting mode. Emission
spectra were obtained, for each compound, under excitation at
the wavelength of the absorption maximum. Fluorescence
quantum yields were measured according to literature pro-
cedures. Cresyl violet or quinine bisulfate in 1 N H2SO4 were
used as standards depending on the emission spectral range.29
The lifetime values were obtained from the reconvolution fit
analysis (Edinburgh F900 analysis software) of decay profiles
obtained using the FLS920 instrument under excitation with a
nitrogen-filled nanosecond flashlamp. The quality of the fits was
evidenced by the reduced χ2 value (χ2 < 1.1).
5-((5-(5-(4-(Methoxy)phenyl)thiophen-2-yl)thiophen-2-yl)-
methylene)-1,3-diethyl-dihydro-2-thioxopyrimidine-4,6(1H,5H)-
dione 6b. Dark blue solid (85%). Mp 263–265 °C. IR(nujol)
ν 1685, 1650, 1556, 1531, 1505, 1486, 1170, 1153, 1105, 1026,
974, 897, 824, 815, 803, 794, 783, 751, 722, 668, 648, 632,
582, 550 cm−1 1H NMR (DMSO-d6) δ 1.22 (m, 6 H, 2 ×
.
NCH2CH3), 3.80 (s, 3 H, OCH3), 4.45 (m, 4 H, 2 × NCH2CH3),
7.00 (d, 2 H, J = 8.7 Hz, 3′′-H and 5′′-H), 7.55 (d, 1 H, J =
3.9 Hz, 4′-H), 7.72 (m, 3 H, 2′′-H and 6′′-H and 3′-H), 7.78 (d,
1 H, J = 3.9 Hz, 3-H), 8.27 (d, 1 H, J = 3.9 Hz, 4-H), 8.64 (s,
1 H,vCH). MS (EI) m/z (%): 482 (M+, 100), 449 (38),
422 (10), 394 (29), 366 (5), 353 (8), 326 (45), 298 (9), 281 (45),
272 (29), 253 (38), 229 (12), 200 (18), 171 (8), 148 (4). HRMS:
m/z (EI) for C24H22N2O3S3; calcd 482.0800; found: 482.0793.
3.3 Two-photon absorption
5-((5-(5-(4-(Dimethylamino)phenyl)thiophen-2-yl)thiophen-2-yl)-
methylene)-1,3-diethyl-dihydro-2-thioxopyrimidine-4,6(1H,5H)-
dione 6d. Green solid (89%). Mp 246–248 °C. IR(nujol)
ν 1682, 1656, 1539, 1513, 1330, 1301, 1258, 1213, 1193, 1151,
1103, 1075, 1059, 1000, 985, 959, 912, 897, 817, 796, 782,
TPA cross sections (σ2) were determined from the two-photon
excited fluorescence (TPEF) cross sections (σ2Φ) and the fluore-
scence emission quantum yield (Φ). TPEF cross sections of
10−4 M chloroform solutions were measured relative to fluor-
escein in 0.01 M aqueous NaOH for 715–980 nm,26 using the
well-established method described by Xu and Webb26a and the
755, 741, 720, 682, 663, 652, 628, 608, 584, 551 cm−1 1H
.
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appropriate solvent-related refractive index corrections.30 Refer-
ence values between 700 and 715 nm for fluorescein were taken
from the literature.31 The quadratic dependence of the fluore-
scence intensity on the excitation power was checked for each
sample and all wavelengths, indicating that the measurements
were carried out in intensity regimes where saturation or photo-
degradation did not occur.
Measurements were conducted using excitation sources deli-
vering fs pulses. This is preferred in order to avoid excited state
absorption during the pulse duration, a phenomenon which has
been shown to lead to overestimated TPA cross-section values.
To span the 700–980 nm range, a Nd:YLF-pumped Ti:sapphire
oscillator was used generating 150 fs pulses at a 76 MHz rate.
To span the 1000–1400 nm range, an OPO (PP-BBO) was
added to the setup to collect and modulate the output signal of
the Ti:sapphire oscillator. The excitation was focused into the
cuvette through a microscope objective (10×, NA 0.25). The
fluorescence was detected in epifluorescence mode via a dichroic
mirror (Chroma 675dcxru) and a barrier filter (Chroma e650sp-2p)
by a compact CCD spectrometer module BWTek BTC112E. Total
fluorescence intensities were obtained by integrating the corrected
emission.
which bear strong D (i.e. NR2) and A (DCV or DETB) end-
groups were found to combine large TPA cross-sections in the
1000–1200 nm spectral range (up to 1600 GM), as well as red or
NIR emission. These spectral characteristics are of major interest
for biological microscopic imaging due to the reduced scattering
they provide in tissues. The push–pull arylbithiophenes combin-
ing strong D and A end-groups thus hold promise for the design
of biphotonic fluorescent probes for bioimaging. We are cur-
rently investigating this route.
Acknowledgements
MBD gratefully thanks the Conseil Regional d’Aquitaine for
financial support (Chaire d’Accueil grant). We also acknowledge
Bordeaux 1 University and CNRS for financial support. The
spectrophotometer, spectrofluorimeter and OPO were purchased
thanks to ATIP and CNRS equipment grants. EG and GC thank
CNRS for a research fellowship (Délégation) and NOEMI
respectively.
MMMR thanks the Fundação para a Ciência e Tecnologia
(Portugal) and FEDER-COMPETE for financial support through
the Centro de Química - Universidade do Minho, through project
PTDC/QUI/66251/2006
(FCOMP-01-0124-FEDER-007429).
The NMR spectrometer Bruker Avance III 400 is part of the
National NMR Network and was purchased within the frame-
work of the National Program for Scientific Re-equipment,
contract REDE/1517/RMN/2005 with funds from POCI 2010
(FEDER) and FCT.
4. Conclusion
The investigation of the photophysical and two-photon absorp-
tion properties in a set of series of push–pull derivatives having
one or two thienyl moieties in the π-conjugated system allowed
us to derive helpful structure–property relationships. These com-
pounds show intense absorption in the near UV-visible region,
as well as two-photon absorption in the NIR region in connec-
tion with a low-lying intramolecular charge transfer excited state.
Interestingly, in such compounds, increasing the length of the
conjugated system as well as the strength of the D end-groups
allows to increase and bathochromically shift both one-photon
and two-photon absorption as well as fluorescence. As a result,
orange to red bright emitting fluorophores were obtained as illus-
trated for example in the cases of compounds 3e (λem = 612 nm
and Φ = 74%) and 4e (λem = 727 nm and Φ = 34%) that
combine strong NEt2 donor end-groups and the phenyl–bi(thienyl)
conjugated path. In contrast, increasing the strength of the A
end-groups also leads to a significant bathochromic shift of both
one- and two-photon absorption and emission as well as an
increase of (one and two-photon) absorption (ICT) bands but at
the cost of a reduction of the fluorescence quantum yield. As a
result when combining strong D and A end-groups, fluorophores
emitting in the NIR region were obtained as illustrated in the
case of fluorophore 6d (λem = 781 nm and Φ = 6%).
All push–pull derivatives show a broad TPA band located in
the NIR region whose position matches well the intense one-
photon absorption band located in the visible region, indicating
that the ICT transition is both one and two-photon allowed. This
TPA band is significantly red-shifted and its magnitude is found
to increase considerably with both the strength of the end-groups
and the length of the conjugated path. Such enhancement has
also been observed in different quadrupolar and octupolar
derivatives with different π-connectors (i.e. vinylene–aryle-
ne).11e,17d–g As a result, push–pull derivatives 4e, 5d and 6d
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